The Collaborative Stepped Care intervention will be compared with Enhanced Usual care in a cluster randomized controlled trial in 24 primary care facilities; the first phase of the trial will be located in 12 government run Primary Health Centres, while the second phase will be located in 12 private family physician/general practice facilities. Approximately 2400 participants who screen positive on a screening questionnaire for CMD will be recruited from the 24 facilities (a harmonic mean of 100 per cluster per arm) of whom 2/3 will be ICD10 cases confirmed by a diagnostic interview (the Revised Clinical Interview Schedule). Follow-up interviews at home will be conducted by an independent team at 2, 6 and 12 months post recruitment. Allowing for 25% attrition during follow-up gives 650 patients in each arm with a confirmed CMD, or a total of 960 in each arm, contributing to analyses of the primary outcomes (see below).